1. Field of the Invention
The present invention generally relates to methods for preparing cathodoluminescent phosphors using a sol-gel condensation technique, as well as to products made from these methods. In particular, the present invention relates to methods for preparing cathodoluminescent phosphors (e.g., orthosilicate-based phosphors) exhibiting superior brightness and efficiency, making them especially suitable for low voltage operation in various applications such as flat panel displays, field emitter displays (FEDs), electroluminescent displays (ELDs), TVs, and the like.
2. Description of the Background Art
Phosphors in general comprise a wide band gap semiconductor matrix with homogeneously dispersed activator ions within. Currently accepted mechanisms of light output in cathodoluminescence phosphors, though not well understood, include electron-induced creation of excitons, which can result in photon emission through recombination of the holes and electrons. However, lattice defects, impurities, charge traps, etc. can impede the efficient recombination of these charge carriers, thus causing the nonradiative decay of the excited states. It is believed that the phosphor crystal structure should be as close to perfect as possible to achieve efficient emission of light.
Current commercially-available cathodoluminescent phosphors are made for high voltage (i.e., approximately 5-20 kV) applications. On information and belief, bright and efficient phosphors that are especially suited for low voltage operation (at or below about 1-6 kV, preferably 2-3 kV) do not exist in the prior art. Thus, it would be very desirable to provide cathodoluminescent phosphors having superior brightness and efficiency at low voltages (e.g., below about 2000 volts) for field emitter displays mainly due to the requirement of the very close proximity of the phosphor screen to the electron source (i.e., the field emitter arrays). Low bias voltages reduce the serious problems of electrical insulation breakdown and arcing.
Many conventional cathodoluminescent phosphors, such as those based on orthosilicates with grain sizes of a few micrometers, are prepared by mixing is micron-sized or larger precursor particles and firing at high temperatures to induce solid reactions. For example, to make green Mn-doped zinc-orthosilicate phosphors, particles of Mn-doped zinc oxide (ZnO) are mixed with SiO2 particles and fired at high temperatures to produce the phosphor compound Zn2SiO4:Mn via solid reaction. The objective of this conventional method would be to cause homogeneous fusion of the precursor components, uniform incorporation of the activator (or dopant) species, and good crystal structure formation. However, due to the slowness of solid fusion/reactions, especially between large particles, good homogeneity is not easily achieved. Lattice defects and even non-stoichiometrical components can result, leading to poor semiconductor electronic band structures, including gap states that can easily cause nonradiative decay. Furthermore, portions that have an activator (e.g., Mn) deficiency can be formed, contributing to a “dead layer” that gives no light output. Other portions can potentially have excess amounts of the activator species which can quench each other, resulting in decreased light output.
U.S. Pat. No. 5,985,176 to Rao discloses Mn2+-activated zinc orthosilicate phosphors having the empirical formula:Zn(2−x)MnxSiO4 wherein 0.005<×<0.15. The phosphors described in this patent are said to exhibit the properties of “improved brightness and decreased persistence” (column 3, lines 3-12) and are made by using the sol-gel process (column 3, lines 13-24 and column 5, line 7 to column 6, line 11). According to the patent, a high degree of homogeneity is achievable because the starting materials are mixed at the molecular level in a solution (column 3, lines 27-29). Unlike the present invention, however, this patent discloses the use of tetraethoxysilane (TEOS) instead of a solid precursor.
Commonly-owned, copending U.S. application Ser. No. 09/398,947, filed on Aug. 2, 1998, which is incorporated herein by reference for all purposes, discloses phosphor nanoscale powder prepared by forming a solution or slurry comprising phosphor precursors and then firing the solid residue of the solution or slurry. In Example I of the '947 application, a mixture of Zn and Mn(II) or Cu(II) precursors (e.g., zinc and manganese(II) acetates) is refluxed in ethanol to obtain a mixed solution of alkoxides/acetates of 1 wt.% Zn, with the amount of Mn being in the range of 1-4% with respect to the weight of Zn. The mixed alkoxide/acetate is then cooled and hydrolyzed with tetramethylammonium hydroxide to form a sol comprising of a suspension of mixed nanoparticles of metal oxides. After that, AEROSIL® fumed silica (7 nm in diameter, Degussa Corporation) is introduced into the sol to form a suspension of the particle precursors. Following ultrasonication, cooling, and drying, the resulting mixed gel is then pre-fired, cooled, ground, and fired. In contrast, in a typical embodiment of the present invention, a Zn-Mn alkoxide solution is first prepared from Zn and Mn alkoxide precursors without hydrolysis or forming particles. In fact, an inhibitor such as nitric acid is typically added to prevent premature (i.e., before introduction of the second precursor particles) precipitation of particles. The second precursor particles (e.g., fumed silica) are then added to the solution of the Zn-Mn alkoxide mixture, followed by induced precipitation of the first precursor by sol-gel condensation reaction, the precipitated first precursor being in intimate contact with and around the second precursor particles.